The chemical treatment of various carbon steel alloys to provide an iron or zinc phosphate conversion coating is very common in the metal finishing industry. These conversion coatings are typically applied by spray or immersion application of an acidic solution containing phosphoric acid as a source of phosphate ions and dissolved metal ions such as iron and zinc. It is generally believed that these phosphate conversion coating compositions react with the ferrous substrate to form a conversion coating of iron/phosphate complexes, zinc/iron/phosphate complexes, or similar metal phosphate complexes. These resulting conversion coatings provide a protective function against corrosion of the ferrous metal substrate and promote the adhesion of subsequent organic coatings such as paints and inks. The bath of the conversion coating composition to which the metal is exposed is typically controlled within a pH range from about 2.5 to 5.5 and at a temperature of about 110 to 160° F. Further, prior art conversion coating compositions typically contained phosphate ions in the range from about 8000 to 20,000 parts per million. With mounting pressure within the finishing to comply with continually tightening process water effluent standards and energy conservation to counter rapid cost increases, the development of new conversion coating technologies have been investigated to reduce phosphate levels and process temperatures at which a bath is operated to provide a suitable conversion coating.
Thus, considerable efforts have been devoted to developing effective conversion coatings for ferrous metals which include organic compounds and film forming compositions. These attempts to develop such conversion coatings include polymeric and other organic coatings, tannic acid and other acidic compositions, and non chromate metal ion solutions. U.S. Pat. No. 4,338,140 to Reghi describes a coating for corrosion resistance with solutions which include zirconium, fluoride and tannin compounds at a pH of 1.5 to 3.5. These compositions also may include phosphate ions. U.S. Pat. No. 4,470,853 to Das describes a coating composition which includes zirconium, fluoride, tannin, phosphate and zinc at a pH of 2.3 to 2.95.
U.S. Pat. No. 5,342,456 to Dolan describes a dry in place coating composition which includes an anion component which has at least four fluorine atoms and at least one of zirconium, hafnium, silicon and boron with optional oxygen atoms; a cation component selected from cobalt, magnesium, manganese, zinc, nickel, tin, zirconium, iron, aluminum and copper, a compound which will form an organic resinous film upon drying in place and a pH of 0.5 to 5.0.
U.S. Pat. No. 6,758,916 to McCormick similarly describes a chrome free dry in place conversion coating composition which McCormick says has a flurometallate anion having at least four fluorine atoms and at least one of titanium, zirconium, hafnium, silicon, aluminum and boron with oxygen atoms; divalent or tetravalent cations of cobalt, magnesium, manganese, zinc, nickel, tin, zirconium, iron, aluminum and copper; an inorganic oxyanion component which has phosphorus; and a water dispersible polymer of hydroxystyrene.
U.S. Pat. No. 4,992,115 to Ikeda describes a surface treatment chemical for aluminum which includes 10-1000 parts by weight vanadium or cerium ion, 10-500 parts by weight zirconium ion; 10-500 parts by weight phosphate ion and 1-50 parts by weight “effective” fluorine ion with a pH of 2-4.0. According to Ikeda, effective fluorine ion means “isolated fluorine” that can be measured with a meter with a fluorine ion electrode. The is apparently contrasted with the sources of zirconium which include zirconium associated with fluorine in compounds such as H2ZrF6.
U.S. Pat. No. 6,027,579 to Das et al. describes a non-chrome rinse composition for rinsing and sealing phosphate conversion coatings. The rinse includes zirconium ions, vanadium ions, fluoride ions and phosphate ions with optional nitrate ions at a dilute concentration for rinsing. Das did not conversion coat, but rather describes a rinse for a conversion coating and does not describe critical ratios of zirconium atoms to fluoride atoms and vanadium atoms to phosphate ions for his rinse composition.
Finally U.S. Pat. No. 6,083,309 to Tomlinson describes Group IV-A protective films for solid surfaces that include aluminum and steel. Tomlinson's compositions include zirconium (as a Group IV-A metal) at a concentration of 1×10−6 moles per liter to about 2.0 moles per liter; at least one anion with a charge-to-radius ration of less than 0.735; not more than 4 fluoride atoms per Group IV-A metal; a pH of less than 5; and water. According to Tomlinson, his compositions are very sensitive to fluoride and he prefers to have no fluoride mixed with his Group IV-A metal as this could cause gelling. Hence his composition is limited to low levels of fluoride relative to the Group IV-A metal such as zirconium. This level is no more than four fluorides to one zirconium.
Some of these alternative non-chrome coatings, particularly those containing organics, are water soluble and may stain or discolor the surfaces of the substrate. Further, non-chrome conversion coating compositions with organics are undesirable because they may interfere or damage the water recycling and reconditioning systems used at metal treatment plants, and may leave residues that interfere with the adherence of paints and other over-coatings.